Aircraft radio altimeter



Aug. 11, 1936. T. J. BOERNER AIRCRAFT RADIO ALTIMETER Filed Jan. so,1955 Patented Aug. 11,- 193e UNI -STAES PTENT AIRCRAFT RADIO ALTDVIETERApplication January 30, 1935, Serial No. 4,127

6 Claims. (Cl. Z50- 1) The present invention concerns a method of andmeans for determining the distance from a moving craft such as anairplane and an object such as the earth or a mountain or another movingvessel. The invention involves the use of radiant energy waves in anovel manner.

Heretofore, systems of the` type involved in the present invention usedthe principle that standing radio waves are produced by reflection fromany conductive reiiecting object when radio frequency energy is radiatedfrom the plane or moving object. The nodes and loops of the standingwaves produced by the reflected and the direct wave has been detected bya receiver on the plane or moving object, thus giving an indication ofthe altitude of the plane from the ground or the distance "from theplane to the mountain or other object. It ls obvious that if such asystem as known heretofore in the art is to be Aeffective, it mustutilize wave lengths of the order of 1,500fmeters or more, because anyattempt by the pilot or operator to count the number of nodes or loopsof the standing wave which he has passed while ascending or descendingresults in 2 confusion, due to the close spacing of said nodes or loops.'I'he useof 1,500 meters requires a bulky.

antenna system and in addition the available frequencies in this portionof the frequency spectrum are congested, so that interference from otherstations may cause erroneous readings.

The object Iof the present invention is to produce a method of and meansfor determining the distance between a moving vessel and any reiiectingobject which is free of the defects inherent 35 in systems of this typeknown heretofore. The method and means of the present invention usesultra high frequency energy which is modulated .or wobbled at acomparatively low frequency, such as 200 kilocycles. The altitudeindication is 40 obtained in accordance with the present invention bymeasuring the phase difference between the local and the reflected 200kilocycle components of the ultra high frequency wave energy. The novelfeatures of my invention have been 45 pointed out with particularity inthe claims ap.

pended to the specification. The nature of my invention and the mannerin which the vsame is carried out will be better understood by thefollowing detailed description thereof vwhen read 50 in connection withthe attached drawing, in

which:

Fig. 1 illustrates somewhat diagrammatically the essential elements of adistance measuring device arranged in accordance with the present 55invention, while Fig. 2 is a graph in which meter deflections areplotted against altitude. The graph serves to illustrate the operationof the present invention.

Referring' to the drawing and in particular to Fig. l, A is a powersupply unit which, as indi- 5 cated, supplies all voltages needed foroperating the equipment. The equipment utilizes thermionic tubes andtherefore this source supplies anode potentials, control grid biasingpotentials,

filament heating potentials, and any other neces- 10 sary potentials orcurrents. The source A may consist of an accumulator or an alternatingcurrent generator and a rectifier or any other known source. Where theenergy is generated on the plane, the generator may be driven by themotive 15 power of the plane or by propeller slip stream energy, or inany other manner known in the prior art. The power supply per se formsno part of the present invention, and the details thereof need not bedescribed herein.

The unit B represents an oscillator preferably of the thermionic type.This unit may also include power amplifiers of the thermionic type. Theoscillator and/or the amplifiers are preferably tunable and Supplyoscillations of substantially constant frequency which frequency may bevaried continuously through a frequency range from 60 to 600 megacycles.

'Ihe high frequency-energy produced in B is acted on by energy from theunit C and is either wobbled, that is, frequency modulated, or modu-`lated in amplitude by a comparatively low freguency wave, such as forexample a Wave of 200 kilocycles. The kilocycle source may be includedalong with the modulator tubes in the unit C and may be of thethermionic type of any other desirable type of low frequency oscillationgenerator. Where amplitude modulation of the high frequency wave isaccomplished, the Heising method of modulation may be used. The carrierwave in B, however, is preferably frequency modulated or "wobbled in anywell known manner by the wave of 200 cycles from C.

The modulated high frequency energy is fed from the transmitter B to adirective antenna system D which, by reason of the short wave lengthused, may be made very compact and highly directive. The unit D may alsobe mounted by way of a universal joint on the plane structure so as toenable the operator or pilot to project the wave in any desireddirection.

The receiving antenna E is also located on the plane and is preferablyspaced as far from the radiator system D as the structure of the planefuselage will permit. The receiver antenna may .receiving antenna E. The

be nondirectional but preferably is adjustable and directive.

The energy picked up by the receiving antenna system E is supplied byway of a balancer F to the high frequency amplifier and detector unit H.The balancer F couples a voltage characteristic of the modulated carrierwave to the receiving antenna. The voltage is made equal in amplitudebut opposite in phase to the voltage induced in the received antenna bythe energy radiated directly from the transmitter antenna. In thismanner I compensate the effect of the directly radiated wave on thereceiver and insure a correct reading. The balancer may be a simpletransformer as shown in Fig. 1 in which a static shield interposedvbetween the tworwpindings so that only electromagnetic and notelectrostatic coupling is obtained.

n The receiving antenna will in practice be located several wave lengthsfrom the transmitter antenna and preferably a phase shifter J isinterposed as shown in the line supplying the balancing of compensatingvoltage from the transmitter to the receiving antenna. 'Ihis phaseshifter may be of any known type. 1n its simplest form the phase shiftermay be as a variable inductive arrangement or any other device capableof varying the length of the transmission line between the transmitter Band the balancer F a range of one-half a wave length.

K represents a variable coupler device by means of which the amplitudeof the balancing or compensating voltage supplied from B to the phaseshifter J may be varied. The coupler K may consist of a simpletransformer in which an electrostatic shield is interposed between thetwo windings and in which the output voltage may be varied by rotatingone coil with respect to the other to increase or decrease the couplingbetween coils. rIhe phase shifter J may consist of a pair ofcontinuously variable inductors, one in each line. These inductorseffectively increase or decrease the length of the line between K and Fas the inductance is increased or decreased respectively.

The high frequency amplifier in H raises the level of the receivedsignal through an ultra high frequency amplifier and demodulates orrectifies he signal by means of a novel thermionic detecor. output fromthe detector passes through a unit L which includes 'an intermediateamplifier by means of which the level of the intermediate frequency waveis increased. The amplified energy is fed from the output of theintermediate frequency amplifier in L to a thermionic limiter whichmaintains the output voltage from L constant regardless of reflectedfield intensity at the limiter in L may be any of several differentforms one of which'is as shown a class C" amplifier with grid biasobtained partly from'its own rectified grid current and partly by meansof a source of grid bias.

The output voltage of the amplifier and limiter in L and a voltage ofthesame amplitude and fre-- quency from unit C are both fed into a phasedifferential detector M which detects the phase differcice between thelow frequency oscillations pro uced in C and the oscillations on thereflected wave. 'I'he phase differential detector in M includes a meterin which the phase difference between`the wave in C and the energycharacteristic of the reflected wave in M is converted into altitude ordistance indicating readings. The reading in M may be calibrateddirectly in feet.

. the wobbler frequency one-halved to 100 kilo- The intermediatefrequency or 200 kilocycle In Fig. 1 I have shown one way in which thephase dierence between the low frequency wave from C and the energyrepresentative of the rei'lected and compensated wave from E may bedetected, compared and recorded. The voltage at 5 the rectifier in Mwill be that obtained by combining the two cut-of-phase sine wavevoltages and will be of the same frequency as eitherwave, but will varyin amplitude from 0 to twice the amplitude of either wave as the phasedisplacement 1o between the two waves varies from 180 to 0. Therectified current in the ammeter A will be proportional to the square ofthe combined voltages. No appreciable phase displacement should beproduced in the wave from C by the equipment when a 200 kilocycle waveis used. However, a phase shifter as shown in Fig. 1 comprising aninductive and capacitive net-work may be used to advantage to impressthe wave from C the grid cathode circuit of the rectifier tube as 20explained below.

If no phase shifter is used and the altimeter A is an ordinary directcurrent milliameter, the deiiections of the pointer of A will follow thecurve of Fig. 2. This condition would probably be objectionable to theoperator, since he is most interested in accurate readings at the loweraltitudes or shorter distances, such as 100 feet. If the meter is biasedY'so that it reads negative 2, 3o cycles, and the initial phasedisplacement adjusted to on the ground, the meter would then read "0 onthe ground and the reading would increase rapidly for a small increasein altitude near the ground, but would taper oil' as the altitude 35approaches 1230. The same effect may also be obtained without the use ofthe phase shifter by using a meter with specially designed pole pieces.

The phase shifter interposed between thesou'rce of low frequencycurrents and the input of the rectifier in M may comprise an artificialline including series inductances and parallel capacities connected asshown. The artificial line may be one-half a wave length long or longerat 200 or kilocycles. A radio goniometer may also be used as the phaseshifter.

For the case assumed above, the altimeter of the present invention willbe useful up to 1230 feet or a one-quarter wave length at 200kilocycles. r It may be made usefu1 from i200 feet to 12,000 00 feet ifthe operator is provided with a control for changing the wobblerfrequency to 20 kilocycles, and the corresponding change in intermediatefrequency tuning all in one operation. Likewise, the instrument may beused as an accurate la.nd ing altimeter eifective up to 1200 feet by asimilar shift of modulating frequency up to 2000 kilocycles. Nospecially designed bandpass amplifier' will be necessary to pass thishigh modulated fre-4 60 quency since the carrier'frequency is from 20 to200 times the wobbler frequency.

The altimeter of the present invention may be used to give a continuousindication oi height about ground regardless of atmospheric condi- 65tions of visibility. It will also give an indication to the pilot as tothe character of the terrain over which he is ying. 'I'he altimeter maybe used as an accurate landing device, as well as a device to indicatehigh altitude. l

The directive radiating and receivingl elements permit the pilot todirect the beam and receive the reflected wave from any direction, asfor as mountains or buildings, or tomoving obstructions, such Vasanother plane :dying the same course of his special direction.

The necessary potentials for all of the tube elements of the tubes usedin the intermediate frequency amplifier and limiter in L and for therectifier in M may be drawn from the main power supply A, or separatesupply sources may be used for any or all of the tubes. For the sake ofclearness I have shown separate sources for the tubes illustrated in Land M. Moreover the main supply unit may be connected to theintermediate frequency amplifier in L to supply all necessary ,powerthereto.

What I claim is:

1.- 'Ihe method of measuring the distance between a source of carrierwave energy and a f reflecting object which includes the steps of, im-

pressing oscillations of relatively low frequency on said carrier waveenergy, directively radiating the resultant energy, receiving saidresultant energy at said source when reflected from said object,limiting the amplitude of said resultant energy, demodulating therceived and amplitudelimited energy, shifting the phase of waves fromsaid low frequency source, combining the demodulated energy with thephase shifted energy from said relatively lowV frequency source, andrectifying the combined energy.

2. The method of measuring the distance between a source of carrier Waveenergy and a reflecting object which includes the steps of, impressingoscillations of relatively low frequency on said carrier wave energy,directively radiating the resultant energy, receiving said energy atsaid` source when reflected from said object, demodulating said receivedenergy, limiting the amplitude of the energy demodulated and comparingthe phase of a component of said demodulated limited energy with thephase of oscillations of like amplitude of said relatively lowfrequency.

3. The method of measuring the distance be- -tween a source of carrierwave energy and a reflecting object which includes the steps of,impressing oscillations of relatively low frequency on said carrier Waveenergy, directively radiating the resultant energy, receiving saidenergy at said source when reflected from said object, impressing onsaid received energy compensating energy from said source of relativelylow frequency, demodulating said received and compensated energy,limiting the amplitude of said demodulated energy and comparing thephase of a component of said demodulated energy with the phase of saidoscillations of relatively' low frequency.

4. In a device for determining the distance between a source of carrierwaves and a reflecting object, a source of relatively low frequencyoscillations, a. source of carrier wave oscillations, a frequencymodulator connected with said source of relatively low frequency, saidmodulator being energized by carrier waves from said source, a directiveradiator connected with said modulator, an absorption member, anamplifier and demodulator coupled to said absorption member, anintermediate frequency amplier and amplitude limiter coupled to saidultra high frequency amplifier and demodulator, a phase detector andindicator coupled to said intermediate frequency amplifier and amplitudelimiter, a coupling between said source of oscillations of relativelylow frequency and said phase detector, and phase shifting means in saidlast named coupling.

5. A device for determining the distance between a source of carrierwaves and a reflecting object comprising in combination a source ofrelatively low frequency oscillations, a modulator connected with saidsource of relatively low frequency oscillations, said modulator alsobeing energized by carrier waves, an antenna connected with saidmodulator, a receiver including an antenna and an amplifier, a couplingbetween said modulator and said antenna, a demodulator and an indicatorcoupled to said amplifier, and a coupling between said source ofoscillations of relatively low frequency and said demodulator, and phaseshifting means in said last named coupling.

6. In a device for determining distance, a source of carrier waves, asource of relatively low frequency oscillations, means for modulatingthe carrier wave energy with energy from the low frequency source, meansfor directively radiating the resultant energy toward an object thedistance to which is to be determined, means for receiving some of thatenergy when reflected back from said object, an amplitude limiter anddmodulator responsive to the reception of said energy, means forshifting the phase of the energy from the low frequency source and forfeeding the same directly to said demodulator thereby to combine th'edirectly propagated energy with the reflected energy, and indicatingmeans responsive to the combined output energy from said demodulator.

THOMAS JAMES BOERNER.

